Ethically? Legally? I've already started building the front-end and it's pretty much complete. Even though the theme/idea of the site will be totally different, should I be having second thoughts?
You shouldn't literally copy/paste anyone's stuff. That's not only unethical it may land you legal trouble.<p>You should take significant inspiration from other people, as long as you re-create it yourself and change it in a meaningful way.
A lot of people heavily borrow from existing site's designs; however, it'll probably hurt your portfolio. If people realize that your designs aren't actually original they may not want to hire you or they may insist on reduced pricing.